In farming, plants, such as maize or beets, which are grown with relatively large spaces between one another, are normally drilled by individual seed drilling machines. A tractor thereby moves several row units, which are attached side by side next to one another along a tool bar, over a field and the row units are controlled in such a way that the seed material is discharged at intervals which are as regular as possible. Since the row units cannot cover the entire field in a single crossing, the tractor must turn at the end of the field after drilling a first strip and must then travel over the field in the opposite direction and discharge the seed material on a second strip of the field.
EP 1 415 523 A1, regarded as being of the generic type, proposes in contrast equipping the individual row units of a drilling machine with an associated drive and controlling these drives centrally by using an external position-determining system in order to obtain a predetermined drilling pattern. The discharge point of the individual seed grains is detected by means of a seed material sensor in order to identify the angular position of a drilling disc of the row unit. Based on these measures, a two-dimensional drilling pattern can indeed be produced, but detecting the actual position of the seed grain is not always sufficiently accurate, since the time between the discharge of the seed grain through the drilling disc and the moment the seed grain reaches its final position in the ground can be different, for example in the case of seed grains of different sizes, and the spatial stagger between the discharge of the seed grain through the drilling disc and its reaching its final position in the ground depends on the travelling speed of the tractor, which is not necessarily constant. Thus with the assembly according to EP 1 415 523 A1, undesired offsets between adjacent plant rows are still possible.
This problem also arises for the assembly according to DE 10 2005 010 686 A1 in which the drives of the cell wheels of the row units are controlled using a position-determining system in order to align the delivery points of adjoining rows with one another, and the drives are synchronized with one another through incremental encoders.
It is furthermore proposed to detect the position of seed grains in the furrow using a seed material sensor and to control the drives of the drilling units so that desired relative spacings of the plants in the travel direction are obtained (EP 2 227 932 A1), or to detect the seed material optically as it falls through the seed material tubes and using a position-determining system and the measured forward speed of the drilling machine to determine the delivery position and to register it in a chart (U.S. Pat. No. 6,941,225).
The invention is concerned with the problem of improving a drilling machine which has at least one row unit to the extent that a desired drilling pattern can be obtained with higher precision compared with the prior art.